The present invention relates to communications systems and methods, and more particularly, to code division multiple access (CDMA) communications systems and methods of operation thereof.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS (Advanced Mobile Phone Service), ETACS (Extended Total Access Communications System), NMT (Nordic Mobile Telephony)-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems, such as those conforming to the North American standard IS-54 and the European standard GSM (Global System for Mobile communications), have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more control channels and one or more traffic channels. A control channel is a channel used for transmitting cell identification, paging and other control information. Traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication for a cell 24. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, while more modern digital wireless systems may use multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency.
CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading sequence, i.e., a sequence that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates. Conventional spread-spectrum CDMA communications systems commonly use xe2x80x9cdirect sequencexe2x80x9d (DS) spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
A direct sequence spread spectrum receiver typically includes a local sequence generator that locally produces a replica of a spreading sequence. This locally generated sequence is used to recover information from a transmitted spread spectrum signal that is modulated according to the same spreading sequence. Before information in a transmitted signal can be recovered, however, the locally generated spreading sequence typically must be synchronized with the spreading sequence that modulates the transmitted signal.
Synchronization of terminals is commonly achieved by transmitting a synchronization signal in each cell that a terminal can acquire to obtain a timing reference for synchronizing its de-spreading operations. For example, in an IS-95 compliant system, a xe2x80x9cpilot channelxe2x80x9d including a fixed carrier modulated by a known sequence is transmitted in each cell of the system, with a respective timing offset applied in a respective cell. In wideband CDMA systems currently under development, such as in WCDMA systems proposed under the 3rd Generation Partnership Project (3GPP), as described in Technical Specification TS 25.213, v2.3.0 (1999-9), available at http://www.3gpp.org, a downlink synchronization channel (SCH) is used to transmit a synchronization (or xe2x80x9csearchxe2x80x9d) code at known times to provide synchronization. In the aforementioned WCDMA system, a primary synchronization code (PSC) is transmitted on a primary SCH once every slot, with the same PSC being transmitted in each cell of the system. The PSC can be detected by a terminal and used to aid the terminal in determining slot timing, as described, for example, in xe2x80x9cPerformance and Complexity of Techniques for Achieving Fast Sector Identification in an Asynchronous CDMA System,xe2x80x9d by Ostberg et al., published in Proceedings of the 1998 Wireless Multimedia Conference, Japan, November 1998. A respective secondary synchronization code (SSC) is transmitted by a respective cell in parallel with the PSC on a secondary SCH, and identifies which of group of scrambling codes is used by the cell.
These synchronization codes are typically non-orthogonal with respect to the other modulation codes used in the system. Although this can periodically destroys orthogonality among signals and can lead to interference with other channels, it is conventionally assumed that channel coding and bit interleaving can mitigate the effects of such interference.
The present invention arises from the realization that, although encoded information, such as information in the data fields of slots of a downlink channel, may be somewhat immune to the effects of synchronization code interference, synchronization code interference can introduce error in such coded information, and less highly coded information, such as power control bits or pilot symbols, can be even more easily corrupted by coincidence with transmitted synchronization code symbols. According to embodiments of the present invention, information is transmitted on a first channel according to a synchronization code, such as a primary synchronization code (PSC) or a secondary synchronization code (SSC). Information is selectively communicated on a second channel to reduce interference attributable to the synchronization code in a signal received over the second channel. Interference cancellation techniques may be used at a receiver to cancel interference associated with the synchronization code. Instead of, or in addition to, such interference cancellation techniques, the second channel may be transmitted using an offset slot structure that is aligned with the first channel such that the synchronization code does not coincide with interference-sensitive information such as power control information or pilot symbols transmitted over the second channel. Offset slot boundaries can also be used in conjunction with multiple alternative slot structures to better distribute power commands while reducing coincidence of the synchronization codes and the power control and pilot symbols. In addition, modified demodulation techniques can be used that lessen interference effects for encoded information transmitted over the second channel.
In particular, in a code division multiple access (CDMA) communications system according to an embodiment of the present invention, a synchronization code is transmitted on a first channel. Information is selectively communicated on a second channel, such as a dedicated physical channel, to limit interference attributable to the synchronization code in a signal received over the second channel.
According to one embodiment of the present invention, information of respective levels of sensitivity to interference is transmitted on the second channel. Information is transmitted on the second channel using a slot configuration that reduces the likelihood of time-coincidence of the synchronization code with first information transmitted on the second channel having a higher level of sensitivity to interference, such as transmit power control (TPC) information or pilot symbols, in comparison to second information transmitted on the second channel. For example, information may be transmitted on the second channel using a slot boundary offset that is selected to reduce the likelihood of time-coincidence of the synchronization code with the first information.
According to another embodiment of the present invention, a signal including the second channel is received. Interference associated with the synchronization code is canceled from the received signal to recover information originally transmitted on the second channel. An estimate of a component of the received signal associated with the synchronization code may be generated based on a channel estimate and knowledge of the synchronization code, and canceled from the received signal to generate an interference-canceled version of the received signal. The interference-canceled version of the received signal may then be processed to recover information originally transmitted on the second channel.
According to another aspect of the present invention, a code division multiple access (CDMA) communications system includes a base transceiver station (BTS) operative to transmit a synchronization code on a first channel and to transmit information on a second channel using a slot configuration that reduces the likelihood of time-coincidence of the synchronization code with first information transmitted on the second channel having a higher level of sensitivity to interference in comparison to second information transmitted on the second channel. In one embodiment of the present invention, the BTS is operative to transmit on a second channel using a slot boundary offset that reduces the likelihood of time-coincidence of the synchronization code with the first information.
According to another aspect of the present invention, a code division multiple access (CDMA) terminal includes a synchronization code interference canceling receiver operative to cancel interference attributable to a synchronization code transmitted on a first channel in a signal received over a second channel. In one embodiment of the present invention, the synchronization code interference canceling receiver includes an RF-to-baseband converter operative to receive a radio frequency (RF) communications signal and operative to produce a baseband signal therefrom, and a synchronization code interference canceling baseband processor operative to cancel a component of the baseband signal associated with the synchronization code. The synchronization code interference canceling baseband processor may include a synchronization code interference signal generator circuit that generates an estimate of a component of the baseband signal associated with the synchronization code based on the synchronization code and a channel estimate, and a summing circuit operative to subtract the estimate of the component from the baseband signal to generate an interference-canceled version of the baseband signal. A demodulator may demodulate the interference-canceled version of the baseband signal to produce estimates of symbols originally transmitted on the second channel.